Type II Transmembrane Serine Protease (TTSP) polypeptides are related, membrane-anchored polypeptides that are involved in cell surface proteolysis and share common structural features including a proteolytic domain, a stem region comprising varying modular structural domains, a transmembrane domain, and a short cytoplasmic domain (Hooper et al., J. Biol. Chem. 276:857, 2001). Members of this family include hepsin (Leytus et al., Biochemistry 27:1067, 1988), enteropeptidase (also referred to as enterokinase; Kitamoto et al., Biochemistry 34:4562, 1995), TMPRSS2 (Paoloni-Giacobino et al., Genomics 44:309 1997), human airway trypsin-like protease (HAT; Yamaoka et al., J. Biol. Chem. 273:11895, 1998), corin (Yan et al. J. Biol. Chem. 274:14296, 1999), MT-SP1 (also known as matriptase; Lin et al., J. Biol. Chem. 274:18231, 1999), and TMPRSS4 (Wallrapp et al., Cancer Res. 60:2602, 2000). Kim et al. (Biochim. Biophys. Acta. 1518:204, 2001) disclose cDNAs encoding proteins with putative serine protease domains and potential regulatory domains; one of the putative proteins also had a transmembrane domain.
The proteolytic domains of TTSPs exhibit a high degree of homology, with highly conserved motifs comprising histidine, aspartate and serine residues thought to be necessary for catalytic activity. A conserved activation motif contains an arginine or lysine, and indicates that the TTSPs are likely to be activated following cleavage. The presence of conserved cysteine residues, and their predicted disulphide bonding pattern, provide support for the belief that TTSPs are likely to remain associated with the cell membrane even after cleavage/activation, although soluble forms of some TTSPs have been identified (Hooper et al., supra). Additional conserved cysteine residues appear to be involved in forming disulphide bonds within the catalytic domain. Cleavage specificities and potential substrates have been identified for some TTSPs, but remain unknown for most; it is likely that the substrate(s) for TTSPs preferentially contain an arginine or lysine in the P1 amino acid position (as originally described for serine proteases in Schecter et al., Biochem. Biophys. Res. Commun. 27:157, 1967).
A hydrophobic, transmembrane domain is present near the N-terminus of the members of the TTSP family, indicating that the proteolytic domain is extracellular. The presence of the catalytic domain on the outside of cell, but presumably still in association with the cell membrane, suggests a role for this family of serine proteases in regulated release of substrate proteins from the cell surface, either from the same cell upon which the TTSP is found or from a cell that is in close association with such a cell.
Most TTSPs exhibit relatively restricted expression patterns, indicating that they may carry out tissue-specific functions (Hooper et al., supra). The variability of the length of the cytoplasmic domains of TTSPs renders it difficult to predict a role for these proteins in cellular signaling; however, some TTSPs do contain consensus phosphorylation sites within the cytoplasmic domain. The greatest degree of variability between members of the TTSP family occurs in the stem region, which may contain up to eleven structural domains. The variety in number and type of structural domains present in the stem region suggests that it may serve to regulate the activity and/or binding of TTSPs to substrates. The role of cell surface proteolysis in homeostasis and disease demonstrates that there is a need in the art to identify and characterize additional members of the TTSP family.